The subject of the invention is a microwave balun for mixers and modulators using TEM wave guides in microstrip realisation.
The balun is an electrical circuit device transmitting radio frequency (RF) signal from an unbalanced input to a balanced output with minimum loses, whereby a broad transmission band and a high impedance transformation ratio are desirable as much as possible. The balun can be applied not only for a balanced load but also for a balanced pair of unbalanced outputs on which RF signals have equal amplitudes but opposite phases.
The balun is a unit which is an integral part of microwave balanced systems such as mixers, modulators, attenuators, switches and others, or a self-contained unit supplying RF signal to balanced loads or inputs or outputs of balanced systems. The mentioned types of systems are very often produced using microstrip lines technique which enables integration thereof in the form of hybrid- or monolithic integrated circuits. This is the consequence of the property of microstrip lines which consists in concentrating the field of electromagnetic wave in the volume under the surface of the strip at a considerable shortening of wavelength in the dielectric substrate and in maintaining planarity of the structure with an easy access to the top surface of the circuit for hybrid attached elements, while the bottom side is fixed as a ground plane. Therefore, it is desirable that the postulate of the microstrip type structure of the circuit should be maintained as close as possible.
From among the hitherto applied baluns two should be distinguished, whereof one is the widely known Marchand's balun, and the other one is the Mouw's balun according to the U.S. Pat. No. 3,818,385. The Marchand's balun consists of two sections. Each section is composed of two coaxial lines, whereof one is placed coaxially in the other. Both sections are connected with each other by conductors between the external coaxial lines. The internal lines are established by two quarter-wave sections of the inner coaxial lines, whereby their internal conductors are connected with each other in the middle and inside of the balun and one of their ends is connected to the input external coaxial line, while the other end remains open to the ground. The internal ends of the external conductors of the said lines are connected to the balanced external output line leading to the balanced load outside the circuit, while their external ends are short-circuited to the ground. The Marchand's balun excellently meets functional requirements both with respect to being broadband and to impedance transformation ratio properties. Its transmission band can exceed even three octaves at loads selected from the range of from 50 Ohm up to 200 Ohm. It is one of the oldest microwave circuits of this type and is still being applied and permanently improved. It is widely applied for feeding the broadband symmetric antennas. It is, however, inapplicable in the balanced systems mentioned previously for the reason of its coaxial type structure and relatively large dimensions, especially if the cross-section is considered. The application thereof compels the use of coaxial type wave guides for systems directly co-operating therewith, which is not always acceptable, and certainly inconsistent with the demand of integration of systems.
The Mouw's balun, improved by Hallford and described in the article "A Designer's Guide to Planar Mixer Balun" published in Microwaves December 1979 pp. 52-57, is a relatively recent design. It consists of three conductive striplines, whereof one--the input one, is formed on the top side of the dielectric substrate, and the other two--the output ones--placed thereunder, are formed on the bottom side of the substrate. The external ends of the output lines are short-circuited to the ground which is established by contiguous metallization plane on the bottom side of the substrate. The Mouw's balun has more applications in balanced systems than the Marchand's balun design. It partially meets the planarity requirement, due to what it is somewhat closer to the microstrip type structure. Although its broadband and transformation properties do not come up to those of the Merchand's design, they are sufficiently good for the needs of designing broadband performance systems exceeding one octave.
The drawback thereof is, however, the necessity of using the lines on both sides of the substrate, whereby the system cannot be at one side fixed to the ground. Since Mouw's design can offer a limited choice of the cross-section dimensions, and in consequence thereof, obtaining the desired performance, it can be applied only for few selected types of balanced systems.
Among other examples not mentioned above, there is a rat race hydrid coupler called also a 180.degree. hydrid, which meets the requirement of both microstrip type structure and the equal division of RF signal into two outpus in opposite phases but its frequency band achieves only 20-30% of the mid-band frequency, and the possibilities of its matching in a wide impedance range are small. Moreover, it has the drawback of inconvenient arrangement of output ports, which hinders using thereof to one balanced load, and its applications are limited to a balanced pair of unbalanced outputs.
The above-discussed examples do not exhaust the existing types of realisation of balun designs but show the already disclosed trend to differentiate considerably the used wave guides from the desired microstrip type, e.g. not only by using coaxial lines but also coplanar-lines, or suspended-lines, or slot-lines, or fin-lines. Realisations of balanced systems in these techniques are plentiful but require forming the system either on both sides of the substrate or on one side thereof, nevertheless, they are always characterized by impossibility of one side bottom fixing the circuit to the ground because the volume beyond the dielectric substrate on both sides thereof and limited by the housing, is an integral part of the wave guide. Such systems are of a big practical importance for structures combined with the waveguide technique but they are not suitable for use in integrated circuits.